1. Field of the Invention
The present invention relates to mobile stations capable of roaming. More particularly, the present invention relates to mobile stations with intelligent control of scan time between home band scans when roaming.
2. Acronyms
The written description provided herein contains acronyms that refer to, for example, various telecommunication services, components and techniques. For purposes of the written description, acronyms will be defined as follows:
Personal Communications Services (PCS)
Personal Communications Network (PCN)
Mobile Telephone Switching Office (MTSO)
Public Switched Telephone Network (PSTN)
System Access List (SIL)
Control Channel (CCh)
System Identification Code (SID)
3. Background Information
The use of wireless or mobile communication devices has increased greatly in recent years. Mobile and cellular telephones have enabled mobile station users to roam over large geographic areas while maintaining immediate access to telephony services. Mobile stations include portable units, units installed in vehicles and fixed subscriber stations. Mobile stations comprising cellular telephones or wireless handsets are operable in cooperation with cellular or Personal Communications Services (PCS) communications systems. Cellular communication systems typically provide service to a geographic area by dividing the area into many smaller areas or cells. Each cell is serviced by a radio transceiver (i.e. a transmitter-receiver base station or cell site). The cell sites or base stations may be connected to Mobile Telephone Switching Offices (MTSOs) through landlines or other communication links, and the MTSOs may, in turn, be connected via landlines to the Public Switched Telephone Network (PSTN).
FIG. 1 illustrates the basic components of a conventional cellular network. As shown in FIG. 1, a mobile station 10 may place or receive calls by communicating with a cell site 12 or a cell site 14, depending upon the geographic location of the mobile station and the cell coverage area that is provided by each cell site. That is, cell coverage area 16 of cell site 12 or cell coverage area 18 of cell site 14. For purposes of illustration, mobile station 10 is depicted in FIG. 1 as being able to communicate with either cell site 12 or cell site 14, even though the mobile station is not illustrated as being located within cell coverage area 16 or cell coverage area 18. Under normal operating conditions, the extent to which mobile station 10 will be able to communicate with cell site 12 or cell site 14 will depend on the geographic location of the mobile station and the size of the cell coverage area of each cell site. Further, although only two cell sites are depicted in FIG. 1, the entire cellular network may include, for example, more than two cell sites. In addition, more than one cell site may be connected to each MTSO.
Mobile station 10 may include a conventional cellular telephone unit with a transceiver and antenna (not shown) to communicate by, for example, radio waves with cell sites 12 and 14. Various air-interface technologies may be implemented to facilitate communication between the mobile station and the cell sites. Cell sites 12 and 14 may both include a radio transceiver (not shown) and be connected by landlines 20 or other communication links to MTSOs 22 and 24. The PSTN 26 is also connected to each MTSO 22 and 24 by landline 20 or other communication links.
The MTSOs 22 and 24 may be conventional digital telephone exchanges that control the switching between the PSTN 26 and the cell sites 12 and 14 to provide wireline-to-mobile, mobile-to-wireline and mobile-to-mobile call connectivity. The MTSOs 22 and 24 may also (i) process mobile station status data received from the cell site controllers; (ii) handle and switch calls between cells; (iii) process diagnostic information; and (iv) compile billing information. The transceiver (not shown) of each cell site 12 and 14 provides communications, such as voice and data, with mobile station 10 while it is present in its geographic domain. The MTSOs 22 and 24 may track and switch mobile station 10 from cell site to cell site, as the mobile station passes through various coverage areas. When mobile station 10 passes from one cell to another cell, the MTSO of the corresponding cell may perform a xe2x80x9chand-offxe2x80x9d that allows the mobile station to be continuously serviced.
In the current cellular system, any given area may be serviced by up to two competing service providers. The two competing service providers are assigned different groups of frequencies through which services are provided. A frequency set typically includes control channels and voice channels. The control channels are used for preliminary communications between a mobile station and a cell site for setting up a call after which a voice channel is assigned for the mobile station""s use on that call. While each frequency set for a given cellular service area is assigned to only one service provider, in different service areas the same frequency set may be assigned to different service providers or companies.
Whenever it places a call, the mobile station will ordinarily attempt to use the home frequency set to establish the call. If a call is handled outside of the user""s home network area, then the mobile station is said to be xe2x80x9croamingxe2x80x9d and service will be attempted through a frequency set of a non-home service provider. Typically, the user""s home service provider will have a roaming agreement or reciprocal billing arrangement with the non-home service provider to permit service to be extended to the user""s mobile station when it is roaming in the non-home service provider""s service area.
When it is roaming in the non-home service provider""s service area during its operation in an automatic mode, the mobile station scans for its home band (i.e., the set of frequencies corresponding to its home network system) to locate a control channel at regular intervals.
According to one aspect of the present invention, there is provided a mobile station capable of quickly returning to a home system of the mobile station from operation to roam within a communication network, the communication network comprising a plurality of cell sites, the mobile station comprising:
a detecting system for detecting, when the home system is not available, signal-level of an available control channel: and
a control system to determine a change in the signal-level,
said control system being operative to control scans of band of the home system in response to said determined change in the signal-level.
According to another aspect of the present invention, there is provided a roaming process for enabling a mobile station to quickly return to a home system of the mobile station from operation to roam within a communication network, the communication network comprising a plurality of cell sites, the mobile station comprising:
detecting, when the home system is not available, signal-level of an available control channel;
determining a change in the signal-level; and
controlling scans of band of the home system of the mobile station in response to said determined change in the signal-level.